1. Field of the Invention
The present invention relates to a display device and a manufacturing method thereof, and particularly relates to a display device having a display panel on which arrayed are a plurality of display pixels having a light emitting element such as an organic electroluminescent element or the like, which has a light emitting function layer formed therein with a liquid material comprising a light emitting function material coated, and a manufacturing method of the display device.
2. Description of the Related Art
Recently, as next-generation display devices that follow liquid crystal display devices (LCD) widely used as monitors and displays of personal computers, video equipment, portable information devices, etc., displays (display devices) having a display panel of a light emitting element type, on which self light emitting elements such as organic electroluminescent elements (hereinafter abbreviated as “organic EL element”) are arrayed in two dimensions, have been vigorously researched and developed so that they can be practically used and become widespread.
Particularly, a light emitting element type display that is adapted to the active matrix drive system has a higher display response speed and no view angle dependency and can obtain a display image with a higher luminance, a higher contrast, a higher preciseness, etc. as compared with a liquid crystal display device. And such a light emitting element type display is advantageous since it can be formed into a slimmer and lighter body because it requires no backlight unlike a liquid crystal display device.
Here, a basic structure of a known organic EL element will be briefly explained, as an example of a self light emitting element used in a light emitting element type display.
FIG. 10 is a schematic cross sectional diagram showing a basic structure of an organic EL element.
As shown in FIG. 10, the organic EL element roughly has a structure where an anode (positive pole) electrode 112, an organic EL layer (light emitting function layer) 113 made of an organic compound, etc. (organic material), and a cathode (negative pole) electrode 114 are sequentially stacked on one surface side (the upper side in the diagram) of an insulating substrate 111 made of a glass substrate or the like.
The organic EL layer 113 has a stacked structure of, for example, a hole transporting layer (hole injection layer) 113a made of a hole transporting material (hole injection layer forming material) and an electron-transporting light emitting layer (light emitting layer) 113b made of an electron-transporting light emitting material. Various low molecular or high molecular organic materials are known as the hole transporting material or the electron-transporting light emitting material to be used for the organic EL layer 113 (the hole transporting layer 13a and the electron-transporting light emitting layer 113b).
Generally, a low molecular organic material imparts a relatively high light emitting efficiency to the organic EL layer, but requires vapor deposition to be applied in its manufacturing process. Therefore, in selectively forming a thin organic film made of the low molecular material only on the anode electrode as the pixel forming region, it is necessary to use a mask for preventing the low molecular material from being vapor-deposited on the regions other than the anode electrode. And since this cannot avoid the low molecular material being adhered even onto the surface of the mask, there is a problem that the material loss is large in the manufacturing process and it is difficult to obtain pixels with high definition.
On the other hand, though a high molecular organic material gives a lower light emitting efficiency to the organic EL layer than given by a low molecular organic material, it can allow the use of an ink jetting method (liquid drop jetting method) or the like as a wet film forming method. Therefore, only the pixel forming region (the region on the anode electrode) can be selectively coated with the solution of the organic material, making it possible to form a thin film of the organic EL layer (the hole transporting layer and the electron-transporting light emitting layer) efficiently and finely.
In the manufacturing process of an organic EL element having an organic EL layer made of such a high molecular organic material, the organic EL layer 113 is formed roughly through the step of forming an anode electrode (positive electrode) on each region (pixel forming region) on which a display pixel is to be formed on an insulating substrate (panel substrate) made of a glass substrate or the like, forming a partitioning wall (bank) made of an insulating resin material or the like on the boundary between adjoining display pixels to define the region surrounded by the partitioning wall as the pixel forming region, and then, with the use of an ink jetting device, coating a liquid material, which is made of a high-molecular organic hole-transporting material dispersed or dissolved in a solvent, on the region and heating and drying the coated region, thereby to form the hole transporting layer 113a shown in FIG. 10, and sequentially through the step of coating a liquid material made of a high-molecular organic electron-transporting light emitting material dispersed or dissolved in a solvent with subsequent heating and drying to form the electron-transporting light emitting layer 113b shown in FIG. 10.
According to the manufacturing method using the ink jetting manner, etc., the above-described partitioning wall has a function of defining each pixel forming region and preventing the phenomenon of the light emitting colors mixing (color mixing), etc. between the display pixels, due to liquid materials of different colors mixing into adjoining pixel forming regions when the liquid materials made of the high molecular organic material are coated. The structure of an organic EL element (display panel) with such a partitioning wall and a manufacturing method using the ink jetting manner for forming an organic EL layer (a hole transporting layer and an electron-transporting light emitting layer) are specifically explained in, for example, Unexamined Japanese Patent Application KOKAI Publication No. 2003-257656.
The organic EL element having the above-described element structure emits light (excited light) VL based on energy that is produced by the recombination of holes and electrons that are injected into the hole transporting layer 113a or into the electron-transporting light emitting layer 113b when a positive voltage is applied to the anode electrode 112 and a negative voltage is applied to the cathode electrode 114 from a direct-current voltage source 115 as shown in FIG. 10. At this time, the light emitting intensity of the visible light VL is controlled according to the amount of the current that flows across the anode electrode 112 and the cathode electrode 114.
Here, by forming either one of the anode electrode 112 and the cathode electrode 114 by using an electrode material having optical transparency, and forming the other of the two by using an electrode material having a light blocking effect and reflectivity, it is possible to realize an organic EL element having a bottom emission type light emitting structure which emits visible light VL toward the other side (the lower side in the diagram) of the insulating substrate 111 as shown in FIG. 10, or an organic EL element having a top emission type light emitting structure which emits visible light VL toward the one surface side (the upper side in the diagram) of the insulating substrate 111.
However, according to the manufacturing method of an organic EL layer (a hole transporting layer and an electron-transporting light emitting layer) using an ink jetting manner, etc. described above, there might occur a phenomenon that the ends of the liquid surface of the coating liquid LQD are pressed up along the side surfaces of the partitioning wall 121 as shown in FIG. 11, due to the characteristic (water repellency) of the surface of the partitioning wall formed to protrude at the boundary between the display pixels (pixel forming regions) and the surface tension of the liquid material coating liquid) made of an organic material. As a result, the organic EL layer is formed thinly at the center region of the region (pixel forming region) where the organic material is coated, while the organic EL layer is thicker at the circumferential region, making the overall thickness of the pixel uneven. Note that FIG. 11 is a schematic diagram for explaining the problem of the manufacturing process of the organic EL element.